N+1 query problem trong JPA là gì? Cách fix?

List<Order> orders = orderRepo.findAll();          // Query 1
for (Order o : orders) o.getCustomer().getName();  // Query 2..N+1 → 100 order = 101 query

// 1. JOIN FETCH
@Query("SELECT o FROM Order o JOIN FETCH o.customer WHERE o.status = :s")
List<Order> findByStatus(@Param("s") String s);

// 2. @EntityGraph
@EntityGraph(attributePaths = {"customer", "items"})
List<Order> findByStatus(String status);

// 3. DTO projection (tốt nhất cho read-only): interface có getId/getStatus/getCustomerName